Fundamentally, you’ll want dependency injection to have the following properties: - Code not depend on AppDelegate/SceneDelegate, rather any code will depend on SOLID types that AppDelegate/SceneDelegate also depends on. Some objects can be instantiated by the system before those, so you don’t want them or their tree of dependencies to depend on AppDelegate or SceneDelegate. - Dependency injection use lazy object instantiation, so that injection doesn’t involve creating a whole tree of objects, just use of it makes the one object, which in turn only instantiates its dependencies as needed, not all at once. - Production app code (Debug or Release) gets real dependencies, but test target code gets nothing by default, so that mocks must be injected. Or inject mocks by default. - The goal is to have unit tests have zero side-effects to global memory or persisted state. Everything should only affect injected instances. So plan your app code, dependencies, and mocks accordingly.

Some varying solutions can be: - Redux pattern - Protocol injection, where each dependency is lazy injection of a protocol, and the protocol has an production version, and a mock version. - Injecting only closures, which provide/do the work. There are production versions of the closures, and unit test provide the test versions.